Well 3 weeks before death is pretty common from people I know who just sorta wanted to see what would happen... all of them were just growing in 'dirt voila' on either a windowsill or under the worng light.
Jsut do a lot of reading in the 'basic' section, which will answer a LOT of questions that you don't even HAVE yet lol!!!
As for the miracle gro soil, it's not really what you want. If it's all you can get, get the 'MG ORGANIC' and add a bunch more perlite to it. Even then it might have too much fertilizer for seedlings under 3 weeks.
Good luck!